<正>Apoptosis is primarily executed by active caspases, which are derived from the inactive procaspase zymogens through proteolytic cleavage. We determined the crystal structures of a caspase zymogen, procaspase-7, and an active caspase-7 without any bound inhibitors. Compared to the inhibitor-bound caspase-7, procaspase-7 zymogen exhibits significant structural differences surrounding the catalytic cleft, which precludes the formation of a productive conformation. Proteolytic cleavage in between the large and small subunits allows rearrangement of essential loops in the active site, priming active caspase-7 for inhibitor/substrate binding. Strikingly, binding by inhibitors causes a 180-degree-flipping of the N-terminus in the small subunit, which interacts with and stabilizes the catalytic cleft. These analyses reveal the structural mechanisms of caspase activation and demonstrate that the inhibitor/substrate binding is a process of